The combined oral contraceptive pill is usually just called "the pill". It contains artificial versions of female hormones oestrogen and progesterone, which women produce naturally in their ovaries.

A woman can get pregnant if a man's sperm reaches one of her eggs (ova). Contraception tries to stop this happening usually by keeping the egg and sperm apart or by stopping the release of an egg (ovulation).

At a glance: the combined pill

When taken correctly, the pill is over 99% effective at preventing pregnancy. This means that fewer than one woman in 100 who use the combined pill as contraception will get pregnant in one year. Other methods, such as the IUD, implant and injection, are more effective.

The usual way to take the pill is to take one every day for 21 days, then stop for seven days, and during this week you have a period-type bleed. You start taking the pill again after seven days.

You need to take the pill at around the same time every day. You could get pregnant if you don't do this, or if you miss a pill, or vomit or have severe diarrhoea.

Some medicines may make the pill less effective. Check with your doctor if you're taking any other tablets.

If you have heavy periods or painful periods, PMS (premenstrual syndrome) or endometriosis the combined pill may help.

Monophasic 21-day pills

This is the most common type. Each pill has the same amount of hormone in it. One pill is taken each day for 21 days and then no pills are taken for the next seven days. Microgynon, Marvelon, Yasmine and Cilest are examples of this type of pill.

Phasic 21-day pills

Phasic pills contain two or three sections of different coloured pills in a pack. Each section contains a different amount of hormones. One pill is taken each day for 21 days and then no pills are taken for the next seven days. Phasic pills need to be taken in the right order. Logynon is an example of this type of pill.

Every day (ED) pills

There are 21 active pills and seven inactive (dummy) pills in a pack. The two types of pill look different. One pill is taken each day for 28 days with no break between packets of pills. Every day pills need to be taken in the right order. Microgynon ED is an example of this type of pill.

Follow the instructions that come with your packet. If you have any questions, ask your GP, practice nurse or pharmacist.

It's important to take the pills as instructed, because missing pills or taking them at the same time as certain medicines may make them less effective.

How to take 21-day pills

Take your first pill from the packet marked with the correct day of the week, or the first pill of the first colour (phasic pills).

Continue to take a pill at the same time each day until the pack is finished.

Stop taking pills for seven days (during these seven days you will get a bleed).

Start your next pack of pills on the eighth day, whether you are still bleeding or not. This should be the same day of the week as when you took your first pill.

How to take every day pills

Take the first pill from the section of the packet marked "start". This will be an active pill.

Continue to take a pill every day, in the correct order and preferably at the same time each day, until the pack is finished (28 days).

During the seven days of taking the inactive pills, you will get a bleed.

Start your next pack of pills after you have finished the first, whether you are still bleeding or not.

Starting the combined pill

Most women can start the pill at any time in their menstrual cycle. There is special guidance if you have just had a baby, abortion or miscarriage.

You may need to use additional contraception during your first days on the pill – this depends on when in your menstrual cycle you start taking it.

Starting on the first day of your period

If you start the combined pill on the first day of your period (day one of your menstrual cycle) you will be protected from pregnancy straight away. You will not need additional contraception.

Starting on the fifth day of your cycle or before

If you start the pill on the fifth day of your period or before, you will still be protected from pregnancy straight away, unless you have a short menstrual cycle (your period is every 23 days or less). If you have a short menstrual cycle, you will need additional contraception, such as condoms, until you have taken the pill for seven days.

Starting after the fifth day of your cycle

You will not be protected from pregnancy straight away and will need additional contraception until you have taken the pill for seven days.

If you start the pill after the fifth day of your cycle, make sure you have not put yourself at risk of pregnancy since your last period. If you're worried you're pregnant when you start the pill, take a pregnancy test three weeks after the last time you had unprotected sex.

Vomiting and diarrhoea

If you vomit within two hours of taking the combined pill, it may not have been fully absorbed into your bloodstream. Take another pill straight away and the next pill at your usual time.

If you continue to be sick, keep using another form of contraception until you've taken the pill again for seven days without vomiting.

Very severe diarrhoea (six to eight watery stools in 24 hours) may also mean that the pill doesn't work properly. Keep taking your pill as normal, but use additional contraception, such as condoms, while you have diarrhoea and for two days after recovering.

Speak to your GP or contraception nurse or call NHS 111 for more information, or if your sickness or diarrhoea continues.

Who can use the combined pill

If there are no medical reasons why you cannot take the pill, and you don't smoke, you can take the pill until your menopause. However, the pill is not suitable for all women. To find out whether the pill is right for you, talk to your GP, practice nurse or pharmacist.

You should not take the pill if you:

are pregnant

smoke and are 35 or older

stopped smoking less than a year ago and are 35 or older

are very overweight

take certain medicines (ask your GP or a health professional at a contraception clinic about this)

After having a baby

If you have just had a baby and are not breastfeeding, you can most likely start the pill on day 21 after the birth but you will need to check with your doctor. You will be protected against pregnancy straight away.

If you start the pill later than 21 days after giving birth, you will need additional contraception (such as condoms) for the next seven days.

If you are breastfeeding, you're not advised to take the combined pill until six weeks after the birth.

After a miscarriage or abortion

If you have had a miscarriage or abortion, you can start the pill up to five days after this and you will be protected from pregnancy straight away. If you start the pill more than five days after the miscarriage or abortion, you'll need to use additional contraception until you have taken the pill for seven days.

breakthrough bleeding and spotting is common in the first few months of using the pill

it has been linked to an increased risk of some serious health conditions, such as thrombosis (blood clots) and breast cancer

The combined pill with other medicines

Some medicines interact with the combined pill and it doesn't work properly. Some interactions are listed on this page, but it is not a complete list. If you want to check your medicines are safe to take with the combined pill, you can:

ask your GP, practice nurse or pharmacist

read the patient information leaflet that comes with your medicine

Antibiotics

The antibiotics rifampicin and rifabutin (which can be used to treat illnesses including tuberculosis and meningitis) can reduce the effectiveness of the combined pill. Other antibiotics do not have this effect.

If you are prescribed rifampicin or rifabutin, you may need additional contraception (such as condoms) while taking the antibiotic. Speak to your doctor or nurse for advice.

Epilepsy and HIV medicines, and St John's wort

The combined pill can interact with medicines called enzyme inducers. These speed up the breakdown of hormones by your liver, reducing the effectiveness of the pill.

The risk of getting a blood clot is very small, but your doctor will check if you have certain risk factors that before prescribing the pill.

The pill can be taken with caution if you have one of the risk factors below. It is unlikely you would be advised to take it if you have two or more risk factors. These include:

being 35 years old or over

being a smoker or having quit smoking in the past year

being very overweight (in women with a BMI of 35 or over, the risks of using the pill usually outweigh the benefits)

having migraines (you should not take the pill if you have severe or regular migraine attacks, especially if you get aura or a warning sign before an attack)

having high blood pressure

having had a blood clot or stroke in the past

having a close relative who had a blood clot when they were younger than 45

being immobile for a long time – for example, in a wheelchair or with a leg in plaster

Cancer

Research is ongoing into the link between breast cancer and the pill. Research suggests that users of all types of hormonal contraception have a slightly higher chance of being diagnosed with breast cancer compared with women who do not use them. However, 10 years after you stop taking the pill, your risk of breast cancer goes back to normal.

How do I change to a different pill?

If you want advice about changing your contraceptive pill, you can visit your GP, contraceptive nurse (sometimes called a family planning nurse), or sexual health clinic.

You should not have a break between different packs, so you will usually be advised to start the new pill immediately or wait until the day after you take the last of your old pills.

You may also be advised to use alternative methods of contraception during the changeover, as the new pill may take a short time to take effect.

If you're under 16 years old

Contraception services are free and confidential, including for people under the age of 16.

If you're under 16 and want contraception, the doctor, nurse or pharmacist won't tell your parents (or carer) as long as they believe you fully understand the information you're given, and your decisions.

Doctors and nurses work under strict guidelines when dealing with people under 16. They'll encourage you to consider telling your parents, but they won't make you.

The only time that a professional might want to tell someone else is if they believe you're at risk of harm, such as abuse. The risk would need to be serious, and they would usually discuss this with you first.